2.1. Hepatitis A Virus
Type A Hepatitis which accounts for approximately 40% of the cases of viral Hepatitis is caused by the Hepatitis A virus (HAV). The host range of HAV is limited to man, apes and several species of New World monkeys (Holmes et al., 1969, Science, 165: 816-817). The virus is spread primarily through fecal and oral contamination.
HAV is an ideal candidate for vaccine development. There is only one serotype and there is little antigenic variation between strains. In addition, immune serum globulin is effective in preventing HAV infection, suggesting the induction of a proper humoral response can be protective. While inactivated and attenuated HAV vaccines have produced protection in primate models (Provost et al., 1986, J. Med. Virol., 19: 23-31), the economy of protection and the risks associated with retained virulence have prevented the use of these strategies to produce commercial HAV vaccines.
The development of HAV epitopic or subunit vaccines has not been highly successful. Ping et al., 1985, Proc. Natl. Acad. Sci. U.S.A., 85: 8281-8285 "Characterization of the Immunodominant Antigenic Site of Hepatitis A Virus", Lemon et al., 1989, Vaccines 89 Cold Spring Harbor, pp. 423-426; Lavener, Charock and Brown, eds., "Identification of the Hepatitis-A Virus Genes Involved in Adaptation to Tissue-culture Growth and Attenuation", in Vaccines 89 Cold Spring Harbor. Although one can produce antibodies (Abs) to denatured proteins, they generally do not neutralize the virus suggesting that major neutralizing epitopes of intact HAV polypeptides may be discontinuous.
The Hepatitis A virus is a 27 nm non-enveloped plus stranded RNA containing member of the enterovirus genus of the picornavirus family (Coulepis et al., 1982, Intervirology, 18: 107-127). The genome of HAV is approximately 7500 nucleotides packaged in a nonenveloped icosahedral capsid and encodes a polyprotein of 2,227 amino acids (Cohen et al., 1987, J. Virol., 61: 50-59). HAV RNA is infectious in cell culture and translation of the RNA yields a polyprotein that is processed to produce a variety of viral structural proteins and enzymes (Locarnini et al., 1981, J. Virol., 37: 216-228; Siegl et al., 1981, J. Gen. Virol., 57: 331-341). The structural proteins include, VP1, VP2, VP3 and possibly VP4, which compose the capsid. In addition, the polyprotein includes proteases, which are presumably involved in the processing of the polyprotein into constituent proteins.
The partial analysis of the sequence of a cDNA clone representing at least 99% of the genome of HAV was described (Ticehurst et al., 1983, Proc. Natl. Acad. Sci. U.S.A., 80: 5885-5889). More recently, the complete nucleotide sequence of an attenuated cell culture-adapted hepatitis A virus cDNA was compared to the cDNA of wild type HAV (Cohen et al., 1987, Proc. Natl. Acad. Sci. U.S.A., 84: 2497-2501; Cohen et al., 1987, J. Virol., 61: 50-59). This full-length cDNA copy of HAV was transcribed and the RNA transfected into monkey kidney cells yields only low levels of HAV (Cohen et al., 1987, J. Virol., 61:50-59).